Early efforts to confront transnational terrorism through national extradition laws created a risk of impunity for egregious offenders, which eventually stimulated greater transnational criminal cooperation and prompted an attempt by the international community to standardize rules in this area. This article considers how the international community has pursued transnational criminal cooperation against terrorism through a variety of legal means: numerous 'sectoral' treaties; efforts to draft a comprehensive international treaty; regional conventions; UN Security Council measures; war crimes liabilities; and debate about an emerging international customary law crime. However, the definition and repression of terrorism involves difficult social and political judgments about who is entitled to use violence, against whom, and for what purposes. Depending on the scope of the international definition of terrorism and any exceptions to it, the criminalization of terrorism risks empowering the state – including autocratic ones – at the expense of other (legitimate) political claims to violence.